There is a
/proc/net/xt_qtaguid/iface/<iface>/{rx_bytes,rx_packets,tx_bytes,...}
but for better convenience and to avoid getting overly stale net/dev stats
we now have
/proc/net/xt_qtaguid/iface_stat_all
which outputs lines of:
iface_name active rx_bytes rx_packets tx_bytes tx_packets
net_dev_rx_bytes net_dev_rx_packets net_dev_tx_bytes net_dev_tx_packets